examples/ecs/iter_combinations: Initialize velocity using fixed timestep (#10673)

# Objective

On some platforms (observed on Windows and Linux, works fine on web),
velocity of all entities would be initialized to zero, making them
simply fall into the star.

## Solution

Using `Time<Fixed>::timestep` instead of `Time::delta_seconds` to
initialize velocity. Since the entities are generated in the startup
schedule, no time has elapsed yet and `Time::delta_seconds` would return
zero on some platforms. `Time<Fixed>::timestep` will always return the
expected time step of `FixedUpdate` schedule.

examples/ecs/iter_combinations.rs

@@ -34,7 +34,7 @@ struct BodyBundle {
 }
 
 fn generate_bodies(
-    time: Res<Time>,
+    time: Res<Time<Fixed>>,
     mut commands: Commands,
     mut meshes: ResMut<Assets<Mesh>>,
     mut materials: ResMut<Assets<StandardMaterial>>,
@@ -81,7 +81,7 @@ fn generate_bodies(
                         rng.gen_range(vel_range.clone()),
                         rng.gen_range(vel_range.clone()),
                         rng.gen_range(vel_range.clone()),
-                    ) * time.delta_seconds(),
+                    ) * time.timestep().as_secs_f32(),
             ),
         });
     }